1. Field of the Invention
The present relates generally to performance improvements in switching circuits and relates more particularly to a reduction in voltage spike magnitude appearing across power switches in a switched power application.
2. Description of Related Art
Class D audio amplifiers have a number of advantages including high efficiency and low cost. A portion of a power stage for an amplifier is illustrated in FIG. 1 as circuit 10. Circuit 10 includes a switching half bridge 12 composed of MOSFETs 13 and 14. In the case of an audio amplifier application, switches 13 and 14 are switched in accordance with a PWM signal related to an input audio signal. Where circuit 10 is used in a high power application such as for a loud speaker, a supply voltage maximum is limited in some cases by a voltage rating of power MOSFETs 13 and 14. For example, the voltage rating of power MOSFETs 13 and 14 may be determined by the values of BVdss and BVii, which are the drain-to-source breakdown voltages of the FET in an on-state and off-state, respectively. The value of BVdss/BVii specifies power MOSFET characteristics for a given power switching application. In the case of circuit 10, switching power MOSFETs 13, 14 are exposed to spike voltages during switching events. The magnitude of the spike voltages depends upon the speed at which current is switched between MOSFETs 13, 14, such as when low side MOSFETs 14 turns off and high side MOSFET 13 turns on, for example. If the switching events occur very rapidly, the switched current can produce high voltage spikes that can exceed the ratings of circuit 10, and damage or destroy MOSFETs 13, 14.
Switching speed for MOSFETs 13, 14 can be controlled by the drive current applied to the gate of MOSFETs 13, 14 by drivers 15, 16 respectively. The drive signals provided by drivers 15, 16 can be applied to the gates of MOSFETs 13, 14 according to a given profile, or with a particular magnitude limit to achieve any given switching characteristic or speed.
In applications that use power switching half bridge configurations, such as DC-DC converters, gate drivers can be used to slow switching speeds of the power switches to avoid high voltage spikes with little or no penalty. In DC-DC converter applications, switching speed is not as critical to the performance of the converter. However, in audio applications with a rapidly changing continuous input, slowing switching speed can have a negative impact on application performance.
If switching speed is reduced, for example, in an audio application, the resulting impact on performance tends to increase the total harmonic distortion (THD) or increase shoot through currents by limiting dead time response. The increase in THD reduces performance with respect to the output of the audio amplifier in terms of sound quality. The increase in shoot through currents tends to impact device longevity and performance because of the high currents experienced by the switching devices. In addition, increases in shoot through currents tend to increase power dissipation, leading to reduced efficiency of the audio amplifier when placed in an idle state.
One solution to the above-described challenges is to specify power switches with higher ratings, which can absorb higher spike voltages without significant impact on the switching device. However, the use of more highly rated switching devices can significantly increase the cost of the audio amplifier solution.
Accordingly, it would be desirable to provide a system and method for handling high voltage spikes in an audio amplifier power stage without modifying switching times of the power stage switches.